Apparatus for amplifying electrical variations



Feb. 20, 1940. Q Q BRQWNE 5 AL 2,190,753

APPARATUS FOR AMPLIFYING ELECTRICAL. VARIATIONS Filed Sept. 14, 1935 4Shets-Sheet 1 Feb. 20, 1940. c, Q BRQWNE r AL 2,190,753

APPARATUS FOR AMPLIFYING ELECTRICAL VARIATIONS Filed Sept 14. 1935 4Sheets-Sheet 2 27 1491225,: 0. Z oW/vg' T86 b TA'E V A "W Feb. 20, 1940.Q Q E 2,190,753

APPARATUS FOR AMPLI FY ING ELECTRI CAL VARIAT I ONS File'i Sept. 14,1935 4 Sheets-Sheet 3 Buick BLACK U UWU C. 0. Wm, T1363 VWEN Feb.20,1940. Q BRQWNE ET AL 2,190,753

APPARATUS FOR AMPLIFYING ELECTRICAL VARIATIONS Filed Sept. i4, 1935 4Sheets-Sheet 4 ,6 Q Eve/126m;

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Patented Feb. 20, 1940 UNITED STATES PATENT orrice APPARATUS FORAMPLIFYING ELECTRICAL VARIATIONS Cecil Oswald Blythen,

Browne, London, and .Frank HayeapEngIand, assignors to Electric &Musical Industries Limited, Middlesex, England, a companyof GreatBritain Application September 14,1935, Serial No. 40,532

In Great Britain S I example, the low frequency and direct currentcomponents represent the average brightness,

and changes in the average brightness; of the object of which an imageis to be transmitted; for convenience in the present specification, anyor all of these components of very low frequency down to zero frequencywill bereferred to as the D. C. component of the variations to beamplified. v It will be understood that the. D. C. component need notnecessarily include a direct current component.

A thermionic amplifier capable of amplifying the whole range offrequencies which electrical variations of the kind referred to mayinclude requires to have conductive couplings between stages and, atleastwhere a high degree of amplification is required, presentsconsiderable practical difiiculties.

In the transmission of electrical signals containing a D. C. componentthrough channels such as amplifiers incapable in themselves i oftransmitting the D. C. component the signals adjust themselves in such amanner that the areas enclosedby the signal wave form above and belowthe so-called electrical zero line are equal. Thus, as the D. C.component ,of the signal varies the absolute values of the signal assumedifferent positions with reference to the electrical zero. This effectis sometimes termed the wander of the electrical zero of the signalsince the effect may be regarded as a wander of the zero withreferenceto the signal wave form, and is also referred to as the absence of a theD. 0. component or in the signals as the absence of the ness component.

It is an object of the present invention to provide novel means wherebyamplification of electrical variations having components down toeffectively zero frequency can be effected with the aid of an A. C.amplifier, that is to plifier incapablein itself of amplifying C,

case of television background bright- In some cases the D. 0. componentof a signal a .may be lost in the apparatus in which the signal isgenerated. An example of this is provided by certain forms of iconoscopeused for thegenera tion of picture signals in a television transmitter,

say, an aim eptember 18, 1934 The picture signals are capacity fed fromthe insulated elements of a mosaic screen to asignal plate in thecircuit of which a load resistance is inserted.- The picture signalcurrents set up diiierences of potential across the load resistance and,in general, these difierences of potential do not contain the D. C.component of the signal.

It is a further object of the present invention to provide novel meansfor obtaining a signal which contains a D. C. component from a signal,generated for example in the manner set forth above, which does notcontain a D. 0. component. The presentinvention accordingly provides amethod of producing from an input signal from which the D. C. componentis absent an electrical output signal, such for example as a televisionsignal, of a desired wave form, said desired wave form being awave formin which a D. C. component or the absolute value of the signal isrepresented and in which, during certain recurrent periods, the signalassumes a datum value,

which method comprises the use of a switching device adapted to convertthe wave form of the input signal to the desired wave form by fixingthevalue of the signal during said periods, said switching device beingrendered operative only during predetermined spaced intervals of time.

The periods above referred to in the above paragraph may be regularlyrecurrent but are not necessarily so. a

For the purpose of the present specification a switching device is takento mean a device which is capable of assuming either a conducting or aninsulating state. Thus the simplest form of switching device is amechanically operated electric switch, but other .forms of switchingdevice, such for example as electron discharge devices, may also beemployed as will hereinafter be explained. Furthermore, the switchingdevice may be of thekind which when in conducting state, only conductsin one direction, in which case the device will be termed aunidirectional switching device. Similarly, a bidirectional switchingdevice is conducts in both directions. Such a device may,

X however, have three states other than the bidione which, whenactuated,

L invention there is provided a method of transmitting an electricalsignal having .a wave form which at spaced intervals of time assumes avalue corresponding to a datum value in the intelligence represented bysaid signal, in which 'said signal is caused, by means of a switchingdevice, to assumean absolute value during said intervals, said switchingdevice being rendered operative intermittently during the transmission.

According to yet another feature of the present invention there isprovided a method of transmitting electrically through a channel whichis incapable in itself of transmitting a D. C. component, signalvariations which include a D. C. component, or which, to representcorrectly the intelligence to which they correspond, should include sucha component, which method'comprises causing the potential or current atthe input of said channel to assume at intervals. a predetermineddatumvalue, and by the aid of means including a switching device, causing thepotentialor current at the output of said channel also to assume atsimilar intervals a substantially fixed datum value, it being arrangedthat said datum potentials or currents exist at the input and output ofsaid channel at the same time.

According to a further feature of the present invention there isprovided a method of transmitting. an electrical signal having a D. 0.component through a channel which is incapable in itself of transmittingsaid component, which method com rises causing the signal fed to theinput of said channel to assume'at intervals a fixed datum value and,during the occurrence of these datum values in output signals from saidchannel, causing said output signals also to assume a fixed datum valueby means of a switching device rendered intermittently operative by anauxiliary signal.

According to yet a further feature of the present invention there isprovided a method for preventing the wander of the electrical zero of anelectrical signal when transmitted through a channel which is incapablein itself of transmitting a D. C. component. which method comprisescausing the signal fed to the input of said channel to assume atintervals a fixed datum value, and, during the occurrence of these datumvalues in output signals from said channel, causing said output signalsvalso to assume a fixed datum value by means of a switching devicerendered intermittenly operative by an auxiliary signal.

Accordin to a still further feature of the present invention there isprovided 'a method for preventing wander of the electrical zero of. orfor givin an absolute value to. an. electrical si nal having a recurrentdatum value. which method come prises brin in said signal to a fixeddatum potential during the occurrence of said. datum value by means of aswitching device rendered intermittently operative by an auxiliarysignal.

' The invention also provides amethod for pre venting wander of theelectrical. zero of. or for iving an absolute value to. an electrical sina having a recurrent datum value which method comprises locking thepotential of the signal to a fixed datum value. by meansof abidirectional switching device, said device only being operative duringperiods whensaid signal is at its datum value. v

The auxiliary signals mentioned above may be derived from thetransmitted signal or they may be transmitted separately therefrom.

The invention further provides apparatus for achieving the objects ofthe invention, and for carrying into efiect the methods set out above.

The invention will be described with reference to the accompanyingdiagrammatic drawings, in

which Fig. 1 shows a simple circuit according to the invention. Fig. 2shows the application of the invention to a television transmitter.

Fig.- 3 shows a modification of a detail of Fig. 2. Fig. 4 illustrates amodification of the arrangement shown in Fig. 2.

Fig. 5 shows a further embodiment of the invention.

Figs. 6 and '7 are explanatory drawings.

Fig. 8 illustrates a modification of the arrangement of Fig. 4.

Fig. 9 is a further explanatory drawing.

Fig. ,10 shows a modification of a detail of Fig. 8, and

Fig. '11 shows an explanatory circuit diagram of .a further embodimentof the invention.

Referring to Fig. 1, signalvariations containing a D. C. component areapplied to a pair of input: terminals I, and it is desired to deriveamplified signals, corresponding to the input signals, from outputterminals 2. The signal volt age is applied through a resistance 3across the terminals of a resistance 4. The potential differences set upacross resistance 4 are applied to the input of an A. C. amplifierrepresented diagrammatically by the rectangle 5, the grid of the inputvalve being shown at E and the anode of the output valve at 1. The anodeI is connected to 'a suitable source of potential (not shown) through aresistance 8, the negative terminal of the source being connected to theconductor 9 and'earth. The potentials developed across resistance 8 arefed to the output terminals 2 through a condenser l0. With thearrangement so far described, the D. C. signal components will not reachthe terminals 2.

In parallel with resistance 4- is arranged a battery' I I in series witha switch It, and in parallel with the output terminals 2 is arrangedanother battery l3 in series with a second switch l4.

Means. not shown. are provided for closing the two switches 12 and I4substantially simultaneously'ior' predetermined short periods and atpredetermined intervals- It will be assumed that battery I I has avoltage V1 and that battery l3 has a voltage V2, where V2 is the voltagewhich would appear in the output in response to a voltage V1 in theinputif the D. C. component were transmitted through the amplifier 5 with thesame amplification as the higher freouency components. It will furtherbe assumed that at a given time under consideration the direct-currentcomponent of the signal has a value such as to make the voltage on thegrid 6 V3 relative to earth. The corresponding output voltage should inthis case be 1 Whenthe switches l2 and I 4 are closed, the voltage ofgrid 6 changes from V3 to V1, a change of V1-Va volts, and there isproduced in the output. due to this change, a pulse of amplitude volts.The pulse of current through the output valve or the amplifier 5 due tothe change in in- The potential of the right .the switch put potentialchanges the potential of the left hand plate of the condenser ID byvolts. At this time, however, the switch I4 is closed and the voltagebetween terminals 2 is determined by the battery It and is equal to V2.

hand plate of condenser H'l is then V2 volts relative to earth. Whenswitches i2 and [4 are opened, the charge on condenser Ill provides atterminals 2 a D. C. potential difference equal to which is the desiredoutput D. C. component corresponding to the input component V3. The timeconstant of the condenser l0 and the circuit associated with the outputterminals 2 is of course arranged to be high enough to ensure thatlittle change takes place in the charge on the condenser In in theintervals between successive closures of the switches 12 and M. Theterminals 2 may conveniently be connected to the control grid andcathode of a further thermionic valve, or the like, in which case theterminals 2 may be regarded as substantially open circuited. It shouldbe noted that although the pulse caused in the input circuit by theopening of i2 produces a corresponding pulse in the output circuit, itdoes not change the charge on the condenser Hi because at this time theswitch M is open and the right hand plate of the condenser is insulated.This pulse occurs during an interval (which may be called thestabilizing interval) when it is assumed that distortion of the signalsis immaterial.

Unless the two switches can be arranged to open and closesimultaneously, it should be arranged that the switch I2 closes justbefore and opens just after the switch [4. The lengths of the periods ofclosure of the switches must be made longer than the reciprocal of thehighest frequency transmitted by the amplifier 5 in order to allow theoutput signal to attain the correct value during each period of closure.

The battery l3 of Fig. 1 is shown for convenience in description onlyand may be omitted, as will be appreciated from what follows; thebattery may provide a potential V2 as in the particular example given,or it may provide any other suitable potential. If battery I3 isomitted, then the lower terminal of switch I4 must be connected toearth. Whatever potential battery l3 may have (the potential may benegative, zero or positive), D. C. will be established at terminals 2,that is to say, zero wander will be eliminated. If the battery has apotential V2 then the output signals are correctly referenced withrespect to earth as datum potential. It will often happen that this isnot the desired condition. For example, if the signals are in thepositive direction at the upper terminal 2 and if this terminal is to beconnected to the grid of a valve having its cathode earthed, then unlessthe upper terminal 2 is maintained more negative with respect to earththan the amplitude of the peak signal, grid current will flow in thevalve and distortion may result. The signals are therefore required tobe referenced about a potential negative with respect to earth by suchan amount that the upper terminal 2 never becomes positive. The correctreference potential is obtained by providing a battery I3 suitablyconnected to provide the desired negative potential.

ture having used in place of the The input circuit of Fig. 1 whichcomprises resistancesfl and 4, battery H and switch I2 is also shown forthe purpose of explanation only. In many cases the function of theswitch l2 will be inherent in the source which supplies signals toterminals I. For example, the scanning of a picblack portions givesintermittent signals of amplitude corresponding to black and these blacksignals may be used as periods of datum value of input. In some scanningsystems the signals in the intervals between successive lines and.frames may be representative of an absolute value of picture brightnessand these intervals therefore provide periods of datum input potential.As long as the output switching device is caused to become conductive atsuitable instants, the method described with reference. to Fig. i may beused to establish or to re-establish D C The arrangement described issuitable for television, code telegraphy and other purposes where thereoccur, or where there can be arranged to occur, periods during which nosignals are transmitted. As has been stated, in the case of television,the intervals between the scanning of successive lines and frames mayconstitute such periods. n

In the arrangement above described, the voltages at the input and theoutput are periodically fixed at. correct relative values. Clearly thecurrents may be fixed instead of the voltages and this will in generalrequire an inductance to be condenser ill, a current of proper valuebeing established in the inductance instead of a proper voltage beingestablished across the condenser.

Instead of either or both of the mechanical switches l2 and M if twoswitches are employed, there may be provided the anode-cathode paths ofthermionic valves, these paths being rendered conducting at suitableinstants by suitable means,

such for example, as positive pulses applied to their grids from asuitable source.

Referring now to Fig. 2, a. cathode ray transmitter i5, which may be ofthe kind described by Zworykin, in the Journal of the Institution ofElectrical Engineers, October 1933, page 437, as an Iconoscope, has itssignal plate it. coupled to the input of an A. C. amplifier H, theoutput of the amplifier being coupled through a condenser l8 to thecontrol grid of a cathode-follower valve 19. A cathode follower valve isone in which the load impedance is connected in the cathode circuit. Theload may be connected between the cathode and earth and the inputbetween the control grid and earth. If then the anode is maintained at asuitable fixed potential, positive with respect to earth, the potentialof the cathode will vary in the same direction as, and to an extentalmost equal to, the variations of potential of the control grid. Thecontrol grid of the cathode-follower valve is is connected to earththrough a resistance 26 in series with a battery 2!. The various leadsin the figure marked with an arrow head are connected to earth, eitherdirectly or through suitable sources of voltage. The cathode (not shown)of the tube i5 is earthed' and the control electrode thereof isindicated by 22. The cathode of the valve i9 is connected through aresistance 23 and a suitable potential source (not shown) to earth. Thegrid of valve 1 i9 is also connected to the anode of a valve 24, thecontrol grid of this valve being connectedto one output terminal of asource of impulses 25, another terminal of the source 25 scanning ofsuccessive v the condenser I8 so as to 4 being connected to the controlelectrode 22. The source 25 is arranged to generate pulses periodically;preferably in the intervals between the lines of the image," and it isarranged that positive pulses are applied to the grid of valve 24 andnegative pulses to the control electrode 22.

The effect of a pulse from the source 25 is on the one hand to make thecontrol electrode 22 negative and so cut off the cathode ray, and on theother hand to make the grid of valve 24 positive and so to make thepotential on the grid of the valve I9 substantially equal to that of thecathode of the valve 24.

The effect of cutting off the cathode ray is to generate a pulse havingan amplitude which is simply related to the signal levels correspondingto full black and maximum white in the picture, and thus to apply adatum potential to the input of the amplifier I1. For a given strengthof cathode ray beam, the value of the datum potential may be found todiffer by a fixed amount from the signal amplitude representing black,and the difference is found to vary somewhat with the average picturebrightness.

The operation of the circuit of Fig. 2 will be seen to be similar tothat of Fig. 1. When an impulse is generated by source 25, there isapplied to the input of amplifier I1 a pulse having an amplitudedependent upon the D. C. component of the picture signals.Simultaneously, the switch constituted by the valve 24 serves to chargebring the grid of valve I9 to substantially the potential of the cathodeof valve 24. The condenser I8 thus receives a charge as in the case ofcondenser ID in Fig. l. The potential of the cathode of valve I9substantially follows that of the grid, and thus the output taken fromacross the resistance 23 contains the D. C. component.

It will be noted that in Fig. 2, the leak is provided for the condenserI8, this leak being taken to a point of such potential that, at eachperiod of switch closure, the condenser I8 always requires to receive acharge in the direction 7 available through the switch valve 24. Thetime constant of the leak 20 with the condenser Ill must be made shorterthan that of the couplings in the amplifier 5 which produce the lowfrequency out-off, but substantially longer than the intervals betweensuccessive switchings.

It may be mentioned that it may not be possible, unless the tube issuitably constructed, to obtain this D. C. component from the tube I5itself. The circuit of Fig. 2, therefore, may serve not only forre-inserting signal components which are lost in an amplifier but mayalso serve to insert components which ordinarily would be absent butwhich are inherent in the signal.

It is desirable that the pulse applied to the control electrode 22 shallbegin before and end after the pulse applied to the grid of the valve 24so that the switch constituted by valve 24 can be regarded as operativeonly whilst the scanning beam is cut off. One reason for this is'that itis practically impossible to arrange that the opening of switch24 andthe cutting-off of the cathode ray beam take place simultaneously andlast for the same period of time; it is clearly undesirable that theswitch 24 should be closed excepting when the beam is cut off. A furtherreason is that the cutting-off of the cathode ray beam may cause thegeneration of spurious signals, probably due to re-arrangement of thecharge on the walls of the Iconoscope I5; these spurious signals make itnecessary to switch on the valve 24 not only after the beam has been cutoff, but also after any spurious signals have ceased. Were the valve 24switched on before the cessation of the spurious signals, and were thelatter of suitable wave form and sufficient amplitude, the valve 24acting as a diode rectifier would give to the right hand side ofcondenser I8 a negative charge; since from the battery 2|, throughresistance 20 to condenser I8 is relatively small, it is possible thatthis negative charge would not be neutralised before the valve 24 openedagain, in which case the potential at the grid of valve I9 would notcorrectly represent the D. 0. component. In Fig. 3 there is shown acircuit whereby it may be arranged that the opening of switch 24 takesplace a suitable interval after the cutting off of the cathode ray beam.The circuit of Fig. 3 is intended to replace parts of the circuit ofFig. 2, as will hereinafter be explained.

Positive impulses from a generator such as 25 in Fig.2, instead of beingled to the valve 24, are applied between the point 26 on a delay network21 and earth; the network comprises shunt condensers and seriesinductances, and is terminated by a resistance 28. The impulses fed tothe point 26 are also applied directly to the control grid of avalve'23. A tapping from halfway along the network 21 is connected tothe control grid of the valve 24 in Fig. 2. The end 3I of the network isconnected to the control gridof a valve 32, and the anodes of the valves29 and 32 are connected together and by a lead 33 to the controlelectrode 22 of Fig. 2. It is arranged that the output from either ofthe valves 29 or 32, taken separately, is suflicient to provide thenecessary bias to the control electrode 22 to suppress the cathode raybeam.

In operation, the leading edge of a positive pulse applied to point 26first reaches the grid of the valve 29 thereby giving to lead 33 thenegative pulse necessary to cut off the cathode ray beam. The leadingedge later arrives at the mid pointof the network 21 and a positivepulse passes along lead 30 and operates the switch valve 24. The leadingedge next reaches the point SI and the grid of valve 32 and thereforethis valve becomes conducting. Valve 29 is already in the conductingstate due to the direct pulse from point 26, and when valve 32 becomesconducting an additional negative pulse is sent along lead 33. Thecathode ray beam is already out off and this pulse therefore has noeffect. The trailing edge of the pulse applied to point 26 then arrivesat the grid of valve 29 and restores the potential of this grid to itsoriginal value. Since, however, the positive pulse from point 3| stillremains on the grid of valve 32, this change in valve 29 has no effecton the cathode ray beam, valve 32 being capable of maintaining the beamin the cut off condition. The trailing edge next reaches the mid pointof network 21 and the grid of valve 24 is restored to its formerpotential, thus opening the switch constituted by this valve. Finallythe trailing edge reaches point 3! and the grid of valve 32. This gridis returned to its original potential, the potential of lead 33 risesand the cathode ray beam is restored. Thus during the interval betweenthe times at which the leading and trailing edges of the positive pulsereach the mid point of network 21, the condenser I3 charges up to suchan extent that the output of valve I9 contains the D. C. component.

the current flowing of Fig. Zmay be worked is as follows. It is An,alternative way iniwhich the arrangement ranged that valve 24 isswitched on during the scanning of the lines oithe picture and D. C. is

dark orfblack signals then cause valve 24 to condenser established withreference to, the black parts of the picture. It foundthat, ingeneraLthere is sufficient blackin ordinary scenes for this purpose.inthis alternative the picture signals must arrive from amplifier I?with black positiveand whitenegative. During the intervals be-l tweenlines it sometimes occursthat, even with the beam switched on", largespurious signalsv are produced. Valve 2 lis; maintained inoperative,

of valve Any true vision signals other than black have no efiect'onvalve 24 since they drive the anode of this, valve negative ,withrespect to its cathode so that valve 24 is effectively insulating as faras the vision signals are concerned. The spurious signals in theintervals between the linescan haveno effect on valve 24 whatever, theiramplitude may be, since duringthese spurious signals, valve 24 isinsulating.

.. It will benoted that in the simple arrangement shown diagrammaticallyin. Fig. 1, current can flow in botli directions through the switch" itwhen it in its closed position, and that acco dire n only through switchvalve 24, andit is dgly necessary to provide a leak for the condenserill so that .the condenser 18 can receivea charge each time switch 2 1closes. There thusa leakage of the D. C. component during the intervalsbetween closures of switch 24.

A modification of the arrangement of Fig. 2,

which employs two-way conducting switches, as

in Fig.1, is shown in Figs. Referring to Fig.4

an A. C. amplifier llreeds a cathode-follower valve is. through acondenser l8, and it is desired to re-insert the D. C. component on thegrid of the valve lil. The lead 34 from amplifier ll passes to a-sourceof signals such as an Iconoscope, and in thiscase. the lead 35 from asource iii of switching impulses passes to the control electrode of theIconoscope.

In placeof the valve 24 and leak of 2 two valves 36 and 31 are employed;theanode of valve of valves 33 and iii, in the positive sense, throughcondensers 36 and 39 respectively; valve 37 is The grid of valveprovided with a grid leak ll].v 38 1s earthed through resistance 4| and.bias battery til inseries, and resistance 4| shunted by diode t3, thefunction of which will be explained hereinafter. 1

The pulses fed to the grid of valve 31 from source it cause: gridcurrent to flow, and thus to charge the condenser:39; the grid of valve31 thus eventually acquires such a negative potential that no anodecurrent flows unless a. pulse present on thegrid.

the charge the correct value.

when theswitch l l open, theright hand side 1035 801111167.1581 ill canbe completely-insulated. On the other hand, in Fig. 2, current canfiowin one The valve 31, when it is conducting, serves as a cathodecircuit impedance for the valve 36, which accordingly functions as acathode follower. The potential to which condenser i8 is stabilizedaccordinglydepends on the datum potential which the grid of valve iscaused periodically to assume,since the cathode of this valve tends toset itself slightly negative relative to the potential or its grid, Thedatum potential may beydetermined bytliesource 2&5, each pulse drivingthe grid oi valve 36 to the same potential, or y by means of the diodell-z, which operates in the following manner: each pulse from sourcecauses current to ilow in .diocle thus, charging condenser toanegativepotentlal which eventually reaches a value such that no anodecurrent flows in valve unless a pulse is present on its grid; each thuscauses the grid of valve 36 to assume the potential of'bias battery t2,and

by. adjusting this potential, control of the potennser l8 is stabilizedcan be tial to whichcond obtained. i i N If when a pulse causes valvesand 3? to pass current, the charge on condenser it is not of the rightvalue, the potential of the cathode of valve 36, and hence of the anodeof valve ill, will in,- crease or decrease slightly and the current invalve 3? will increase or decrease so as to give It can be arranged thata small change in the potential of the oathode of valve causes alargechange in the current flowing in valve ill, so that the potential ofcondenser 58 is very rapidly stabilized.

The valves and 3'3 are arranged to be conducting during what have beenreferred to as stabilizing intervals; the lattenin the case oftelevision signals for example, may be theintervals between trains ofpicture signals, and it must be arranged that no anode current passes invalves and other than in the stabilizing intervals. It is also importantthat the flow of anode current in valvestt and 3? should cease It may bearranged that the valve 3i is 0a slightly before vaiveilt ir' desired,since the cutting-on of valve ill only causes a slight change in thepotential of the cathode of valve 3&5. Alter? natively, itmay bearranged that the amplitudes of. the pulses which are applied to thegrids of valves 36 and 3i are so related to one another that thepotential of the cathode of valve Elli does not vary substantiallyduring the shutting-off of valves 38 and 3?.

The valves 36 and 33' may have similar characteristicsand may besupplied with similar operating potentials, in Which case it may bearranged thatthe grid of valve Bl is always biased slightly negative;this may be achieved by connecting the anode of a diode to the grid ofvalve 31 and biasing the cathode of the diode slightly negative relativeto the cathode of valve ill; in this case, it may be arranged that bothvalves 38 and ii are shutoff at substantially the same rate, in whichcase the cathode of valve tt will remain at a substantially constantpotential during the shuttingofi of the valves.

Fig. 5, to which reference is now directed, shows a circuit which isparticularly suitable for use in component with reference to the peakamplitudes of the synchronising pulses. In Fig. 5, it is assumed thatthe synchronising pulses are in the positive sense. Were thesynchronising pulses in the negative sense, it would be necessary toinsert a reversing valve in the connection between the A- C. amplifierl1 and the control grid of the stabilising valve 46; the arrangement,however, is shown for synchronising pulses positive. The valve 56 is atetrode of the" type in which there is sufiicient secondary emissionfrom the anode to ensure that the anode current is negative for valuesofanode potential. slightly less than the screen potential, that is to"say the anode current-anode voltage characteristic crosses the baseline. The screening grid is given a positive potential relative to thecathode by means of battery 41. The amplifier I1, is an A. C. amplifierto the input of which are applied signals comprising trains'of picturesignals having synchronising pulses in intervals between them, thesynchronising pulses being assumed to be in the blacker-than-blacksense; the synchronising pulses from amplifier I'l serve to switch onthe valve 46 through the connection to its control grid. For normalsignal amplitudes, the control grid is very negative and the valveHis-inoperative. When a synchronising pulse occurs, the control grid ofvalve- 55 is driven to approximately zero potential, the condenser 44and leak 45 serving to bias the control grid automatically. The valve 45becomes conductive and stabilises the potential of itsanode to a valveadjacent to the potential of the screen and representing the point onthe anodecurrent characteristic where the anode current is zero. Themechanism of the D. C. re-establishment can best be followed byreference to the two Figures 6 and '7.

Had there been employed for re-insertingthe D. C. component aconventional diode together with a suitable leak a certain amount ofcharge on the condenser it would have leaked away during each linebetween the synchronisingpulses. For a full white picture, the resultantwave form would have appeared as in Fig. 6 ;-in this figure and in Fig.7' signal voltage or current is plotted against time as abscissa;Referring to Fig. 6 in which parts of the signal above theline markedblack represent picture signals, the slope of the line representingwhite during each line is due to the leaking away of the charge: Thisslope can be made as small as desired by suitably increasing the leak,but-if sudden changes of brightness are to be followed correctly thisincrease involves the construction of the amplifier I! so that itcanpass lower frequencies than would otherwise have been necessary;Furthermore, it is seldom possible to make a diode which has such lowresistance that the charging of condenser i8 is completed during a shortsynchronising pulse. With this incomplete charging, when using framepulses of longer duration than the line pulses, an irregular effect isproduced during the frame pulses which give the diode a better change tocharge the condenser fully, thus producing a slightly irregular-D40.

re-establishment. t

Wth the arrangement shown in Fig. 5, there is no leak, so that for afull white picture, the curve obtained (Fig. 7) is quite fiat during theintervals between synchronising pulses; in fact, if the natural leakageacross condenser I8 is zero, thenthe valve 46 will only have topassanode current when there is a change in average brightness, that is,a change in the D. C. component.

Reference will now be made to Fig. 8, which shows an arrangement whichis a modification of that shown in Fig. l, and which has in common withthe arrangement of Fig. 5 thefeature-that the re-establishment of D. C.is carried outwith reference to fithefl peaks 'of the synchronisingpulses in the arrangement of.

' valves 52'and-53 are so adjustedthat the multivibrator oscillates atline frequency, giving a positive pulse: on valve52 which'is slightlyshorter than the line synchronising pulse, that is to say, the condenserand leak in the grid circuit of valve 52"has' a shorter time'constantthan the condenser and-leak in the grid circuit of valve 53 amulti-vibrator' of this form is described in co-pending application No.7,730/35'. Positive pulses are iedf-rom the anode of valve 52 to thestabilizing valves 54 and 55 these latter valves Fig. 8; the controlpulses'are generated by a multi-vibrator actuated operate in a-similarmanner to valves 36 and 31 in,Fig.-4. it 1 It will be seen that there isa direct'conductive connection between the anodeof valve'52 and the gridofvalve 55, and that the anode of valve 52 is supplied by a suitablebattery 56, the negative terminal of which is connected to earth. Thepotential of this battery decides the potential to which the output willbe stabilised, because when the anode of valve 55 is positive, the valve52 is cut 'ofi and itsanode assumes the potential of the battery 56, andso does the grid of valve 55, which I is conductively connected to theanode of valve 52. The control grid-of valve 54 is biased automatically'to beyond cut-off by means of a .condenser 57 and leak 58, thegrid biasing itself to approximately zero potential relative to itscathode dring the positivepulse from multi-vibrator 52, 53.The'cathodesof valves 51, 52 and 53 are held negative with respecttoearth by means of suitable biasing meansinot shown), since their anodepotentials are approximately earthy. With synchronising pulses 'inthe:positive sense, the

amplitude of voltage swing of theanode of valve 52in re'sponseto asynchronising pulse must exceed'thepeakamplitude of these impulses, sothat valves" 54 and 55-are cut off for all other signal amplitudes.Thesy'nch'ronising pulses are fed to valve 5| through acondenser 59,and. a leak is provided; Signals fed to the grid of valve 5| cause gridcurrent to flow and thus condenser 59 become'sflcharged,thegrid of valve5| thus re quires such anegative potential that no anode ,r

current flows unless a synchronising signal is present-on the grid. Inthis way the synchronising'signalsare separated from the picturesignals. If the' synchronising signals from the amplifier I! are in thenegative sense, a reversing valve should be inserted between amplifierI1 and valve 5L,

-.By -setting' the multi-vibrator to deliver positive pulses to valve 55which are slightly shorter than the synchronising pulses, which latterserve to provide a datum potential at the input of amplifierJJ-git isarranged that the valves 54 and 55 arewell shut off beforethe end of thesynchronising pulses. Furthermore, bysetting the condensers and leaks inthe grid circuits of valves 52 and 53 :so' that the .multi-vibratoroscillates naturally. at line frequency, it can be arranged that themultievibrator oscillates steadily at line frequency .during the:transmission of frame pulses in the form of broadened line pulses, so

, 55 is quite regular during the [frame interval.

terval.

that the o. c. re-establishment by valves 54 and The value of condenserl8 can be made as large as desired provided that valves 54 and 55 arecapable of charging it sufilciently during one synchronising pulse toallow for any required correction. The natural resistance looking intothe cathode circuit of valve 55 may be, say, 150 ohms or less, whichallows a very effective charging of I condenser 68.

In an arrangement in which D. C. is restored with reference to peakamplitudes, a two-way switching device being employedit may be arrangedthat the device remains permanently conductive in one direction andbecomes alternately conductive and non-conductive inthe oppositedirection. The device may of course comprise two paths in parallel, thefirst being permanently conductive in one direction and other directionand the second being permanently insulating in one direction (thedirection in which the firstis conductive) and being associated withmeans for causing it to become alternately conductive and insulatinginthe opposite direction.

A further example of the use of the present invention is illustrated inFigs. 9 and 10, Fig. 9 being an explanatory figure in which signalvoltage or, current is plotted against time as abscissa. Fig. 9 shows awave form where after each line synchronising pulse 1 there is a shortinterval b of black. If desired, the D. C. component may bere-established with reference to this black in- This may be achieved bymeans of a circuit such as is shown in Fig 10 which is'a modification ofthe arrangement cf Fig. 8, it being arranged however, that the multivibrator 52, 53 makes valves 54 and 55 active during the short blackinterval .1). In Fig. 10 the valve 5| operates in a manner similarto theoperation of valve 5| of Fig. 8 and serves to separate thesynchronisingpulses as before, that is to say, its

griel assumes approximately zero potential during the synchronisingpulse, but the valve is cut off for allotheramplitudes. In its anodecir-.

cuit, however, is a small condenser Bl feeding a resistance 62. Thecondenser 6| serves to produce differentiated synchronising pulsesacross the resistance 62. At the beginning of the synchronising pulse, asharp at the top of resistance 62, whereas at the end of thesynchronising pulse a positive pulse appears 'at this point. Thispositive pulse operates on the grid of valve 63 causingits anode tobecome negative and pull the screen grid of valve 52 negative,

thus triggering the multi-vibrator. Themultivibrator is now set toproduce a positive pulse on the anode of valve 52 during the blackinterval 11 in Fig. 9. The remainder of the circuit is as shown in Fig.8.

In the arrangements described with reference to Figs. 5 and 8, it willbe seen that the synchronising impulses serve to apply a datum potentialto the input of the A. C. amplifier. It is assumed that thesynchronising signals have been added to the picture signals at instantswhen the picture signals were at datum value, or at some value fixedlyrelated, to datum value, or have been caused artificially to assume suchdatum values, so that the synchronising impulses all cause the input toassume substantially the same datmn potential. Clearly, any othersuitable means, such as those employed in the arrangements of Figs. 1and 2, may be employed for providing the input datum potential.

.It has been stated earlier in this specificainsulating in the negativepulse appears for rendering said switching conductive in onedirectionand the conductive in the other direction, and means for ticnthat, instead of adjusting the charge on a condenser in order toestablish D. C., the current in .an inductance may be adjusted for thispurpose. Fig. 11 is an explanatory diagram illustrative of the principleof current stabilisa tion. In Fig. 11 a high impedance source 64, suchas the output of an AC. amplifier or the like, has a high inductance 65connected across datum potential, for example, by means of pulsesapplied to it in a manner such as has already been described. Duringthese periods the current in the inductance 65 is forced to assume thevalue of the output current of the source 64 corresponding to the datumpotential and, due to the high value of the inductance, remains substantially at this value during the periods when the switching device 65 isin the insulating state. When device 66 is in the bi-directionallyconducting state, the current in output resistance 61 is determined by"the difference between the instantaneous current from source 54 and thedatum current in inductance 65.

The invention has been particularly described, 1

by way of example, with reference to amplifiers for television signals,but it will be apparent to those versed in the'art that it is not so.limited, but is applicable to many other systems in which it is.required to amplify signals having a D. 0. component, but where the useof a direct coupled amplifier is impracticable or inconvenient and alsoto systems in which it is required to obtain a signal containing a D. C.component from a. signal which has an absolute D. C. significance butwhich does notcontain a D. C. component.

We claim: a

1. Apparatus for producing from an input signal from which the directcurrent coniponentis absent, an output signal in which the directcurrent component is represented, said apparatus comprising a pathincluding a resistance and a condenser in series, means for causingsignal currents to flow in said path in response to said input signals,a unidirectional switching device comprising a thermionic vacuum tube inparallel with said resistance, operating means for actuating saidswitching device intermittently at predetermined datum levels, means forbiasing the input of said tube, means for biasing theoutput of saidtube, and means for substantially simulratus comprising a condenser; abi--directional switching device so connected and arranged as to becapable, when-conducting, of influencing the charge on said condenser, asource of intermiti tent auxiliary signals, and means for feeding saidauxiliary signals to said switching device device bi-directionallyconductive intermittently.

3. Apparatus according to claim 2, comprising a switching deviceconstituted by two conductive paths in parallel, one of said paths beingother being Ill causing said auxiliary signals to render one of saidpaths alternately conductive and insulating.

4. The method of producing a waveform having an absolute value of directcurrent component from a waveform containing none or a less .thanabsolute value of direct current component which comprises the steps ofstoring electrical energy to a voltage corresponding to the value of theabsolute value of the desired direct current component, raising thevalue of the original wave to a predetermined datum wave, amplifying theoriginal wave form at the predeterminable datum level and combining, theamplified wave with the stored energy toproduce the desired wave formhaving an absolute value of direct current component. w .5. The methodof producing a wave form having an absolute value of direct currentcomponent from a waveform containing none or a less than absolute valueof direct current'component which comprises the steps of storingelectrical energy to a'voltage corresponding to the value'of theabsolute value of the desired direct current component, raising thevalue of the original wave to a predetermined datum level, amplifyingthe original wave form at the predeterminable datum level and combiningthe amplified wave at recurrent intervals with the stored energy toproduce the desired wave form having an absolute value of direct currentcomponent.

6. The method of producing a wave form hav:

ing an absolute value of direct current component from a wave formcontaining none or a less than absolute value of direct currentcomponent which comprises the steps of storing electrical energy to avoltage corresponding to the value of the absolute value of the desireddirect current component, raising the value of the original wave to apredetermined datum level, amplifying the original wave form at thepredeterminable datum level and simultaneously combining the amplifiedwave with the stored' energy to produce the desired wave form having anabsolute value of direct current. component.

7. The method of producing awave form having an absolute'value. ofdirect current .component from a wave form containing none or a currentcomponent, raising the value of the original wave to .a predetermineddatum level, am-

plifyingtheoriginal wave form at the predeterminable datum leveldeveloping auxiliary con,- trol signals and combining the-amplified wavewith the stored energy to produce the desired wave form having anabsolutev'alue of direct current component in accordance with thecontrol by the generated control signals.

8. Apparatus for producing from an input signal from which the directcurrent component,

is absent, an output signal in which the direct current component isrepresented, said apparatus comprising a path including a resistance anda condenser in series, means for causing the signalcurrent to flow insaid path in response to said input signal, a unidirectional switchingdevice in parallel with said resistance, means for actuating saidswitching device during periods in which said input signal has a datumvalue, and means for actuating said switching device during periods inwhich said input signal differs from a datum value in such a sense as tobe incapable of actuating said switching device.

9. Apparatus'for producing from an input signal from which the directcurrent component is absent, an output signal in which the directcurrent component is represented, said apparatus comprising a condenser,a bi-directional switching device so connected and arranged as to becapable, when conducting, of influencing the charge on said condenser, asource of intermittent auxiliary signals, and means for feeding saidsauxiliary signals to said switching device for rendering said devicelei-directionally conductive only during periods in which said inputsignal has a datum value. I

' i CECIL OSWALD BROWNE'.

FRANK BLYTHEN.

